The 36-year-old, who began his career at Lazio, will join the club after receiving his International Transfer Certifcate.
He played 78-times for Italy and lifted the World Cup in 2006 after a penalty shootout win over France.
He is the fourth Italian at the club, following in the footsteps of Marco Di Vaio, Bernardo Corradi and Matteo Ferrari.
"I'm very happy to be here,” said Alessandro Nesta. “Despite some offers to continue my career in Europe, I was looking for a new challenge and wanted to play in MLS. Upon visiting my friend Marco (Di Vaio) to see him play his first game with Montreal, I met great people in this club. I really liked what I saw and I was convinced Montreal was where I wanted to be."
"For a player who won the World Cup and played at the highest level for 20 years, he was extremely humble and it was easy to come to an agreement with him," said Montreal Impact president Joey Saputo. "He already feels a part of the team."